Make a cute candy vending machine that only dispenses treats when you knock the secret rhythm on its front panel.
One of the best things about exhibiting at Maker Faire is giving attendees a challenge. For the 2010 Maker Faire Bay Area, I decided to combine a past project of mine, a door lock that opens only when you give a secret knock, with a standard crowd pleaser: candy.
The result was this Secret-Knock Gumball Machine, which tempted and tested the crowds at Maker Faire to guess the right rhythm and receive a treat. Since the knock was not terribly secret (I happily handed out hints), it distributed hundreds of gumballs over the event’s two days.
The “secret” knock defaults to the famous “Shave and a Haircut” rhythm, but you can program custom knocks by simply pressing a button and knocking a new pattern. The machine only listens for the rhythm, not the tempo, so the correct knock will dispense a treat whether you perform it fast or slow.
Inside the machine, a piezo sensor picks up sounds from the front knock panel, while an Arduino microcontroller recognizes the target pattern and controls a servo-driven gumball-dispensing wheel. You can build the Secret-Knock Gumball Machine with its inner workings visible or hidden, depending on whether you want to show off the mechanism or keep it a mystery.
Copies of downloadable files can also be found at http://makezine.com/25/gumball.